Corona free pinning of extruded polymer film

ABSTRACT

The adhesion or pinning of an extruded film forming polymer to a moving quenching member is improved, in the absence of corona discharge, by maintaining the quenching member at an electrical potential while the extrusion die is grounded, one magnitude of said potential being sufficient to increase the adherence of the film to the quenching member.

This application is a continuation of Ser. No. 492,184 filed July 26,1974, now abandoned; which was a continuation of Ser. No. 347,667 filedMar. 30, 1973, now abandoned; which was a continuation of Ser. No.88,555 filed Nov. 12, 1970, now abandoned.

The present invention relates to a process for producing polymeric filmby the extrusion of a thermoplastic film forming polymer.

A common method for producing thermoplastic polymeric film is to extrudethe polymeric material while molten through a flat or circulat extrusiondie. After extrusion, the resulting molten or plastic polymeric film iscast upon a moving cooling or quenching surface constituted by a cooleddrum or belt, where the film is cooled sufficiently to solidify it. Inorder to obtain a rapid quenching of the film, it is important that theheat transfer from the film to the quenching surface be high.

Good heat transfer may be obtained when the film is securely adhered tothe quenching surface by depositing electrostatic charges to the uppersurface of the molten film from a corona-discharge station, prior to thepoint where the film first contacts with its lower surface the quenchingsurface which is electrically grounded. The industrial application ofthe described process is hampered by the phenomenon which is inherent toionization discharging, viz. the tendency to change into arcing undercertain circumstances of relative humidity, air pressure, etc. As aconsequence thereof the film will be punctured or ruptured.

In addition, the corona discharge electrode tends to collect oligomerswhich are set free during the extrusion, so that the ionization maybecome non-uniform along the length of the electrode.

The present invention provides an improved process for producingpolymeric film, which operates without corona discharge, and yet obtainsa good adhesion of the extruded film to the quenching member.

The process for producing film according to the present inventioncomprises extruding a thermoplastic, film-forming, polymeric material inthe form of a continuous film from an extrusion die onto a movingquenching member while maintaining an electrical potential differencebetween said quenching member and some other part which is contacted bythe polymeric material before it reaches said member, the magnitude ofsuch difference being sufficient to improve the adherence of said filmto said quenching member, and withdrawing the solidified film from saidquenching member.

By way of example an electrical potential difference can be establishedbetween the quenching member and the extrusion die. Processes will bedescribed hereinafter wherein the molten polymeric material afterleaving the extrusion die travels in contact with a control surface(e.g., a control roller surface) before reaching the quenching member.In such a process the required results can be achieved by maintaining anelectrical potential difference between the quenching drum and saidcontrol surface; in those circumstances there may also exist anelectrical potential difference between the quenching member and theextrusion die, and/or between the extrusion die and the control surfacebut that is not essential.

The process can be carried out according to one simple embodiment, byconnecting one terminal of an electric power supply to the quenchingmember and the other terminal to the extrusion die or some other part incontact with the polymeric material flowing towards such member. The die(or other said member) and the quenching member, may be electricallyinsulated from each other or there may be some flow of current betweenthem, e.g., via the extruded molten film. In a preferred embodiment, theextrusion die is electrically grounded and the quenching member iselectrically insulated with respect to its mountings, i.e., from theground, and is connected galvanically to one terminal of a source ofD.C. potential. As mentioned already hereinbefore, the quenching memberwith may be a roller, a belt or the like. In the case of a roller, forinstance, the roller may be integrally electrically conductive, or itmay comprise an insulating body portion which is provided with anelectrically conductive peripheral portion. As a further alternative thequenching member may have an electrically conductive body portionprovided with a thin electrically insulating peripheral surface layer.

The improvement of the present invention may be practiced in the contextof the process disclosed in commonly assigned U.S. Pat. No. 4,006,729which issued Jan. 3, 1978 on Ser. No. 506,574, which was a continuationof Ser. No. 319,015, now abandoned, which in turn was a continuation ofSer. No. 43,484, now abandoned, and corresponds to British Pat. No.1,316,411. Those applications relate to a method for making polymericfilm by extrusion wherein molten polymeric material is extruded througha slot orifice to form a film, the extruded film is longitudinallystretched by being drawn away from the extrusion orifice faster than theextrusion speed and is received on a quenching surface and, between theextrusion orifice and the quenching surface, the extruded film makescontact over its full width with a cooled control surface, whichabruptly arrests the necking-in of the film.

As described in the specifications of those earlier applications theremay be more than one quenching member. For example there may be aplurality of quenching drums or other quenching members arranged inseries so that the film passes from one to the other of them. The term"quenching member" as used in defining the present invention thus usedbroadly so as to include any one of two or more such members if morethan one such member is present. In such a case, in which there is morethan one quenching member, the two or more members being arranged inseries, each of the members may be at a potential different from that ofsome part contacted by the polymer flowing towards the first of saidmembers but in any case it is preferable for at least the first of thesaid members to be at a said difference potential.

An additional advantage of the process according to the invention isthat only a small current (if any) is required in the electric circuitincorporating the source of E.M.F. In case there is a closed circuit,current limiting means may be incorporated in such circuit for limitingthe current to a maximum value of about 1×10⁻⁶ A. Moreover, there isvirtually no danger to human operators carrying out a process accordingto the present invention, as a result of electric discharge because, incase the extrusion die is grounded, the capacity of the quenching memberin relation to the ground is so small that, even for a D.C. potential ofone thousand volts, the energy of the quenching member amounts only to avalue of about 1×10⁻⁶ joules. Finally, it is possible to avoid even aslight shock in the event of the quenching member is touched byproviding the lateral sides of the quenching member, and optionally alsothe surface thereof which is contacted by the extruded film, with a thinelectrically insulating layer.

The invention will be described hereinafter with reference to theaccompanying drawings wherein:

FIG. 1 is a diagrammatic view of the extrusion arrangement operatingaccording to the method of the present invention,

FIG. 2 is a front view, partly broken away, of the quenching roller,

FIG. 3 is a diagram representing the influence of the applied potentialdifference across the width and the edge thickness of the cooled film,

FIG. 4 is a diagrammatic view of an alternative extrusion arrangement.

Referring to FIG. 1, molten synthetic thermoplastic film formingpolymeric material is extruded by an extruder 11, forced through afilter 12 and a conduit 13 to the extrusion die 14, and through theorifice 15 of the die onto a positively driven quenching drum 16,rotated by means not shown. Between the quenching drum and the extrusiondie a potential difference is supplied from a D.C. power supply 17. Thesolidified film is drawn off the quenching drum, e.g. over a non-drivenroller 18, and may be submitted to further treatments to improve itsphysical properties. Such treatments are known in the art and mayinclude longitudinal and transverse stretching, heat-setting,post-stretching, heat-relaxing, etc.

While FIG. 1 shows the principle of carrying out the process accordingto the present invention, FIG. 2 adds some more practical details.

The quenching drum 16 is a stainless steel hollow member, the peripheralsurface of which is highly polished and has a mirrorlike finish.

The drum has two fixed shaft extremities 20 and 21 which are rotatablyjournalled in ball bearings, as shown. The ball bearings are pressedinto bushings 22 and 23 of an electrically insulating material, such asTeflon (Registered Trademark) or the like, and the bushings in theirturn are fixedly clamped in the parts 24 and 25 of the machine frame.The shaft extremity 20 is coupled via an insulating coupling 26, alsomade of Teflon, to the drive shaft 28, whereas the shaft extremity 21 ishollow and communicates through a rotatable seal with the inlet 27 andthe outlet 29 for cooling liquid. The inlet and the outlet are connectedby flexible insulating conduits 30 and 31 to an arrangement, indicatedby the arrow 32, and comprising a circulation pump and a cooling group.

The cooling liquid used in the arrangement described was a mixture of 2parts by volume of paraffin oil and 1 part of trichloroethylene. Themixture showed a specific resistivity of 10¹¹ ohm.cm.

The power supply 17 is a D.C. power supply capable of delivering hightension voltages ranging between 0 and 5000 volts. One output terminalof the supply is grounded, whereas the other terminal is connected tothe electrically conductive connection 34 of the conduits of coolingliquid, and in that way is galvanically connected to the quenching drum16. The D.C. output of the power supply used in the present arrangementwas electronically stabilized so that in fact it shows a low internalresistance. Therefore a protective series resistor was provided in thesupply, shown as a separate resistor 33 in the figure, resistor 33having a resistance of 10⁹ ohms in the example which will be describedhereinafter.

The extrusion die 14, the idler roller 18 and all other parts of theinstallation are electrically grounded.

The following example illustrates the invention.

A polyethylene terephthalate polymer having an inherent viscosity of0.59 dl.g⁻¹ and a specific density of 1.33 g.cm⁻³ is extruded at a rateof 60 kgh⁻¹ and at a temperature of 290° C. through a conventional flatextrusion die having an opening of 300 mm by 200 mm. The molten polymeris received on a quenching drum having a diameter of 800 mm and a widthof 1100 mm, which is rotated at a rate of 2.28 m.min⁻¹ and is maintainedat a temperature of about 30° C.

The effective length of the molten film between the extrusion orificeand the first line of contact with the drum amounts to 20 mm.

The thickness of the film in the transverse direction is measured afterremoval of the solidified film from the quenching drum and is plotted inFIG. 3, in which the curve 38 represents the thickness d of the film inrelation to its width when the D.C. potential of the source was adjustedto 1000 volts, and curve 39 indicates the film thickness, when thepotential was reduced to zero.

It appears from the diagram of FIG. 3, that the application of thepotential difference causes the film width to increase (value a) byabout 6 mm from its value (value b) when there is no potentialdifference. This increase in the film width implies a reduction ofnecking-in which means that the film edges have become thinner so thatthe edge loss, i.e. the quantum of film material where the filmthickness exceeds the central film thickness by 1% and which must betrimmed off, is reduced.

The effect of the applied potential difference on the so-called"Venetian blind haze" defect, i.e. a type of haze which is characterizedby a plurality of alternate clear and hazy lines and is believed toresult from entrapment of air between the film and the quenchingsurface, was evaluated visually.

When the potential difference was zero, the mentioned defect was sostrong that after biaxial stretching, heat-setting and heat-relaxing, afilm was obtained with a surface quality which although acceptable forthe production of radiographic film, was totally unacceptable for theproduction of photographic film for graphic purposes.

When the potential difference was 400 volts, the mentioned defect hadalmost completely disappeared except for a plurality of small "hole"defects which were distributed at random at an average density of abouttwo per cm². These hole defects could be characterized as small circularcavities in the film surface which was in contact with the quenchingdrum, having a diameter of about 0.1 mm and an undeterminable smalldepth.

When the potential difference was increased to 500 volts, the Venetianblind haze had completely disappeared and the hole defects were slightlyreduced in number and in size.

As the potential difference was progressively increased, the hole defectdecreased correspondingly until at potential difference of 1000 volts avisually perfectly smooth film was obtained.

In the operation of the extrusion arrangement described, a D.C. currentwas measured in the connection of the power supply to the quenching drumof 0.1×10⁻⁶ A, so that the actual voltage on the quenching drum amountsto 900 volts (i.e. 1000-10⁹ ×0.1×10⁶).

The current flowing through the mentioned connection is actually the sumof three currents flowing separately to the ground. The first currentflows from the quenching drum through the molten film to the groundedextrusion die. The second current flows through the film, removed fromthe drum, to the grounded roller 18. The third current flows fromquenching drum through the bearing insulations, and through the coolingcircuit to the grounded machine parts.

The second current is negligible in respect of the first one, becausethe length of the free film path to the roller 18 is several timesgreater than the length of the free film path to the extrusion die 15,and the conductivity of the cooled film is much lower than the moltenfilm.

The third current is negligible also, because the electric insulation ofthe quenching drum in respect of the ground amounted to 10¹² ohms. Inthis connection it should be mentioned that the electrically insulating,as well as the thermally conductive, properties of the described coolingliquid mixture, remained satisfactory over long periods of use.

From the above it may thus be concluded that the electric conductivityof the molten film is the main parameter which determines the current inthe electric circuit. This conclusion does not hold when there is nogalvanic contact between the film and the quenching drum, such as occurswhen the drum surface is covered with an electrically insulating layer.Such layer may be formed by the deposition of oligomers, or it may bedeliberately provided in order to prevent the operator from touching thecharged drum. Although the current in the circuit is virtually reducedto zero by such insulating layer, it has been found that the favourableeffect of the electric potential difference on the adherence of the filmto the quenching drum remains unaffected.

It should be understood, however, that the presence of such insulatinglayer on the drum surface will generally have an adverse effect on theheat transfer, so that it is preferably neither applied nor allowed toform.

In connection with the electrical danger formed by the describedarrangement, it can be mentioned that the operator could touch thequenching drum at a potential of 900 volts without experiencing anyshock. This is explained by the presence of resistor 33 in the electriccircuit which limits the maximum current to 1×10⁻⁶ A, and the electricenergy stored in the drum which amounts only to 1×10⁻⁶ joules (1/2CV²=1/2·2×10⁻¹² ×10⁶, wherein C is the capacity of the drum versus theground and V is the potential difference).

The invention is not limited to the described embodiment. The polarityof the D.C. power supply may be reversed without any adverse effect onthe process.

The power supply may provide an A.C. voltage but in that case thefrequency of the A.C. voltage must be not so high that most of theelectric energy is lost in undesirable capacitive coupling with machineparts. Moreover, the frequency of the A.C. should differ from the properresonance frequency of the molten film curtain in order to notdisadvantageously influence the process.

The potential difference may also be applied by electrically groundingthe quenching drum and by connecting the extrusion die 14 to the powersupply, the flexible connection 13 providing in that case an electricinsulation between the extrusion die and the other, grounded, extruderparts. Alternatively, the quenching drum and the extrusion die may bothbe electrically insulated from ground and connected each to a terminalof the power supply, the grounding occurring at a potential betweenthose of the power supply terminals.

The means for cooling the extruded film may comprise more than onequenching member as mentioned already in the introduction of thedescription, and in that case all of such members may be maintained at apotential differing from that of the extrusion die. If a cooled controlroller with a small diameter, as described in the above-identified U.S.Pat. No. 4,066,729, is made to contact the extruded film at its concaveside between the extrusion die and the quenching drum, this roller maybe connected to the electric power supply in order to give that controlroller a potential different in magnitude or in magnitude and polarityfrom that of the quenching drum. Such control roller may also have apotential different from the extrusion die as well as from the quenchingdrum, and such an embodiment is illustrated in FIG. 4.

A film 44 which is extruded from an electrically grounded die 14 isdrawn over a free rotating control roller 40 by a positively drivenquenching drum 41 which rotates at a peripheral velocity which isseveral times higher than the speed of the film being extruded from thedie. The control roller is cooled and has a high heat conductivity, sothat the stretching of the molten film is confined to the gap betweenthe extrusion orifice and the control roller. The film is carried on thesurface of the drum 41 for an extended portion of its travel and is thenfed to a second driven quenching drum 43. A roller 42 covered with aresilient layer presses the film on the periphery of the drum 43 in viewof the proper advance of the film. Finally, the film is drawn away overa drum 47 to a further destination.

The rollers 40 and 41 are mounted in electrically insulated fashion andare galvanically connected to electrically grounded D.C. supply means 45and 46. The polarity of one supply is inverted with respect to theother, so that the difference in potential is highest between thecontrol roller 40 and the quenching drum 41.

As an alternative, the control roller may be at a potential equal tothat of the quenching drum and the required electrical potentialdifference may be established between the extrusion die on the one handand the quenching member on the other.

Of course it is possible to ground the control roller and to maintainthe quenching member at a difference potential and the extrusion die canthen be at a potential equal to or different from that of the controlroller.

Finally, it should be understood that the method according to theinvention may be used with success in the extrusion of films of otherpolymeric materials which are cooled rapidly upon extrusion. Somerepresentative materials are polyethylene, polypropylene, copolymersthereof, polystyrene, polyvinylchloride and others.

We claim:
 1. A process for producing polymer film by the stepsconsisting essentially of:(a) extruding a molten thermoplastic, filmforming, polymeric material from an electrically grounded extrusion dieto produce a continuous film, (b) continuously delivering said extrudedfilm onto the surface of a rotatable electrically conductive coolingcylinder electrically insulated from ground, and establishing anintimate adherence between said film and said cooling cylinder surfaceat the initial locus of contact of the film with the roller byconnecting said roller to a source of electrical potential, saidpotential being of sufficient magnitude to increase the adherence of thefilm to said roller surface but insufficient to produce a coronadischarge in the vicinity of said locus, and (c) removing the film fromsaid cooling roller after the film remained in contact therewith for adetermined angular extent.
 2. A process according to claim 1, whereinthere is at least one quenching drum of larger diameter than saidcylinder for cooling the film after removal from said first coolingcylinder, said quenching drum being likewise mounted electricallyinsulated from ground and connected to a second source of electricalpotential.
 3. A process according to claim 2, wherein the polarity ofsaid second source of electrical potential is opposite to the polarityof said first source of potential.
 4. A process according to claim 2,wherein the time during which the film remains in contact with saidquenching drum is substantially greater than the time during which thefilm remains in contact with said cooling cylinder.
 5. A processaccording to claim 2, wherein the film remains in contact with saidquenching drum over an angular extent which is substantially greaterthan the angular extent of contact with said cooling cylinder.
 6. Aprocess according to claim 1, wherein the current flowing from thesource of electrical potential towards said cooling cylinder is limitedto a maximum of about 1×10⁻⁶ A.
 7. A process according to claim 1,wherein said polymeric material is polyethylene terephthalate.